An inkjet printing system typically includes one or more printheads and their corresponding ink supplies. A printhead includes an ink inlet that is connected to its ink supply and an array of drop ejectors, each ejector including an ink pressurization chamber, an ejecting actuator and a nozzle through which droplets of ink are ejected. The ejecting actuator may be one of various types, including a heater that vaporizes some of the ink in the chamber in order to propel a droplet out of the nozzle, or a piezoelectric device that changes the wall geometry of the ink pressurization chamber in order to generate a pressure wave that ejects a droplet. The droplets are typically directed toward paper or other print medium (sometimes generically referred to as recording medium or paper herein) in order to produce an image according to image data that is converted into electronic firing pulses for the drop ejectors as the print medium is moved relative to the printhead.
Motion of the print medium relative to the printhead can consist of keeping the printhead stationary and advancing the print medium past the printhead while the drops are ejected. This architecture is appropriate if the nozzle array on the printhead can address the entire region of interest across the width of the print medium. Such printheads are sometimes called pagewidth printheads. A second type of printer architecture is the carriage printer, where the printhead nozzle array is somewhat smaller than the extent of the region of interest for printing on the print medium and the printhead is mounted on a carriage. In a carriage printer, the print medium is advanced a given distance along a print medium advance direction and then stopped. While the print medium is stopped, the printhead carriage is moved in a carriage scan direction that is substantially perpendicular to the print medium advance direction as the drops are ejected from the nozzles. After the carriage has printed a swath of the image while traversing the print medium, the print medium is advanced, the carriage direction of motion is reversed, and the image is formed swath by swath.
Inkjet ink includes a variety of volatile and nonvolatile components including pigments or dyes, humectants, image durability enhancers, and carriers or solvents. A key consideration in ink formulation and ink delivery is the ability to produce high quality images on the print medium. Image quality can be degraded if air bubbles block the small ink passageways from the ink supply to the array of drop ejectors. Such air bubbles can cause ejected drops to be misdirected from their intended flight paths, or to have a smaller drop volume than intended, or to fail to eject. Air bubbles can arise from a variety of sources. Air that enters the ink supply through a non-airtight enclosure can be dissolved in the ink, and subsequently be exsolved (i.e. come out of solution) from the ink in the printhead at an elevated operating temperature, for example. Air can also be ingested through the printhead nozzles. For a printhead having replaceable ink supplies, such as ink tanks, air can also enter the printhead when an ink tank is changed.
In a conventional inkjet printer, a part of the printhead maintenance station is a cap that is connected to a suction pump, such as a peristaltic or tube pump. The cap surrounds the printhead nozzle face during periods of nonprinting in order to inhibit evaporation of the volatile components of the ink. Periodically, the suction pump is activated to remove ink and unwanted air bubbles from the nozzles. This pumping of ink through the nozzles is not a very efficient process and wastes a significant amount of ink over the life of the printer. Not only is ink wasted, but in addition, a waste pad must be provided in the printer to absorb the ink removed by suction. The waste ink and the waste pad are undesirable expenses. In addition, the waste pad takes up space in the printer, requiring a larger printer volume. Furthermore the waste ink and the waste pad must be subsequently disposed. Also, the suction operation can delay the printing operation
Methods of degassing the ink in an inkjet printer that have previously been disclosed include a) reducing the pressure in an air space in contact with ink, b) heating the ink to cause air bubbles to come out of solution, or a combination of a) and b). U.S. Pat. No. 4,340,895 discloses heating the ink in an ink supply vessel of a recirculating ink supply and using a vacuum pump to provide a negative pressure on an air space above the liquid ink, thereby reducing the amount of gas dissolved in the ink. The ink can then be cooled before being used for printing. Disadvantages of this method include the additional space, cost and noise associated with a vacuum pump as well as the pump for the recirculating ink supply; the excessive energy required to heat the ink; and the need to either cool the ink or print with ink at elevated temperature.
U.S. Pat. No. 5,341,162 discloses heating ink to cause air bubbles to come out of solution in a secondary tank in a recirculating ink supply and enter an air space above the ink. The air then passes through a semi-permeable membrane, permitting air but not liquid to pass through a vent. Disadvantages include the need for a pump for the recirculating ink supply, as well as requiring excessive energy to heat the ink.
An air extraction device is described in commonly assigned US patent application (docket 95796). Such an air extraction device uses a compressible member (which can be compressed using motion of the carriage in a carriage printer, for example) to expel air through a one-way relief valve, thereby applying reduced air pressure at a membrane that is permeable to air but not to liquid. This causes air bubbles to come out of solution and pass through the membrane, with a portion of the accumulated air being expelled during the next compression of the compressible member. Such an air extraction device is satisfactory, and can be operated either with or without heating the ink. However, it requires time and carriage motion in order to compress the compressible member, and compression of the bellows can produce an audible sound.
What is needed is a degassing device for degassing ink in an inkjet printer that can remove air with little or no wastage of ink, that is compatible with a compact printer architecture, that is low cost, that is environmentally friendly, that is quiet, that does not heat the ink appreciably, and that does not delay the printing operation.